For rubber compositions used for a breaker as a tire component, balanced improvement of properties such as fuel economy, hardness, elongation at break, and adhesion to cords (e.g. steel cords) has been desired.
Known methods of improving these properties include methods employing various butadiene rubbers such as modified butadiene rubber and butadiene rubber containing 1,2-syndiotactic polybutadiene crystals, together with natural rubber.
However, completely mixing natural rubber and butadiene rubber during kneading is difficult, and therefore lumps of butadiene rubber may remain in the mixture. Butadiene rubber also has a possibility of exhibiting scorch during synthesis. A blend of natural rubber and butadiene rubber therefore has a problem that the blend has very low tensile strength compared to natural rubber, is likely to be easily peeled off the cords, and is far from having a fail-safe concept in terms of both kneadability and rubber scorch in synthesis.
Rubber compositions for a breaker also contain stearic acid in order to achieve high hardness through the crosslinking reaction of rubber. However, the adhesion of such rubber compositions has not been studied in detail, which means that the above balance of properties has not been considered enough.
Patent Document 1 teaches a composition containing a certain benzothiazolylsulfenimide and a compound such as a cobalt salt of an organic acid; still, the composition still leaves something to be desired in terms of improving the fuel economy, hardness, elongation at break, and adhesion in a balanced manner while satisfying the fail-safe concept.    Patent Document 1: JP 2009-7549 A